read-print: 'pretty-print-with-comments' keeps newlines on long strings.

* guix/read-print.scm (printed-string)[preserve-newlines?]: New
procedure.
Use it to preserve newlines on long strings.
* tests/read-print.scm: Add test.

guix/read-print.scm

@@ -420,11 +420,18 @@ particular newlines, is left as is."
 
 (define (printed-string str context)
   "Return the read syntax for STR depending on CONTEXT."
+  (define (preserve-newlines? str)
+    (and (> (string-length str) 40)
+         (string-index str #\newline)))
+
   (match context
     (()
-     (object->string str))
+     (if (preserve-newlines? str)
+         (escaped-string str)
+         (object->string str)))
     ((head . _)
-     (if (memq head %natural-whitespace-string-forms)
+     (if (or (memq head %natural-whitespace-string-forms)
+             (preserve-newlines? str))
          (escaped-string str)
          (object->string str)))))
 

tests/read-print.scm

@@ -194,6 +194,11 @@ expressions."
 (test-pretty-print "\
 (string-append \"a\\tb\" \"\\n\")")
 
+(test-pretty-print "\
+(display \"This is a very long string.
+It contains line breaks, which are preserved,
+because it's a long string.\")")
+
 (test-pretty-print "\
 (description \"abcdefghijkl
 mnopqrstuvwxyz.\")"